High speed, forced vortex pumps have been known since at least the early 1940's when they were investigated by Dr. U. M. Barske for use in rocket propulsion systems but they have not found wide commercial use since, probably because of their unfamiliar pumping characteristics. Physically, they somewhat resemble the common centrifugal pump but they operate on altogether different principles. A centrifugal pump uses a screw-shaped or scrolled impeller to force all the fluid which enters the pump to be thrown outwardly into an annular discharge channel. Since the fluid moves quickly through the pump, the residence time for any particular portion of the fluid is very short, often less than one revolution of the impeller, thus there is a considerable difference in relative speed between the fluid and the impeller. The characteristics of such pumps are generally well-known and they are commonly used to supply very large flows of fluids at low to moderate pressures.
In contrast, a forced vortex pump (not to be confused with a liquid-ring pump) is based on rapidly rotating a body of fluid and withdrawing only a relatively small portion of the fluid so that the remainder may be considered, for design purposes, almost as a rotating solid body. In its original form, such a pump consisted of a rotating drum with baffles or blades fixed to its inside walls for developing the rotating body. Fluid entered the drum through its hub and was picked up near its periphery by a stationary, internal pickup tube which exited the drum through the hub. Difficulties with adapting this design for various applications led to an inverted design in which a simple, straight impeller with long blades was used to create a rapidly rotating fluid vortex within a short cylindrical cavity within a fixed housing surrounding the rotating impeller. The outer portion of the fluid vortex, adjacent the smooth housing wall, is at a high pressure while the inner portion is at a much lower pressure. Typically, the high pressure fluid is extracted from the housing through a tangential diffuser section where much of the kinetic energy (velocity) of the fluid is converted to static or potential energy (pressure). The pressure level at the discharge is determined by the diameter and rotational speed of the impeller, while the maximum output flow rate is directly related to the size of the diffuser throat at any given rotational speed. Very small, simple pumps can put out moderate flows at high pressure if all the components are carefully designed. In addition, these pumps can operate satisfactorily at very low input pressures, close to the vapor pressure of the fluid, without cavitation. More importantly, the output pressure is practically constant for all rates of flow at any given speed and the output capacity is approximately proportional to the impeller speed (up to a maximum value determined by the number and size of the discharge). This characteristic is sometimes a disadvantage. For example, one potential application of such pumps is in a fuel supply system for expendable gas turbine engines for missile propulsion. However, in such a system the pump would preferably be driven at a fixed speed determined by the rotational speed of the turbine engine (i.e., direct drive) rather than at a variable speed (i.e., separate pump motor) determined by fuel flow required. Such systems would then necessarily include a separate high pressure, hydromechanical fuel control unit to regulate the amount of fuel delivered to the engine. Such control units are usually very complex and expensive. What is needed is a simple and inexpensive way to vary the pump output directly.
In view of the foregoing, it should be apparent that there is a need in the art for improvements in the design and construction of output flow controls for high speed vortex pumps.
It is therefore an object of the present invention to provide an improved method and apparatus for controlling the output flow rate of a vortex pump used to supply large flows of fuel to gas turbine engines.
A further object of this invention is to provide a highly reliable but low weight fuel supply system for gas turbine engines.
It is another object of the invention to provide a simple, low cost, electrically actuated flow control device for a fluid pump operating at a relatively fixed rotational speed.